YouTube High Definition for Firefox allows you to easily play all YouTube videos in HD and more.
It will also permit you to change the video player size, auto stop videos, auto-mute videos, control embedded videos, and turn off annotations. The auto stop function is nice as I regularly scare me at 6 A.M. when the video begins to play right out of the gate - with the volume at 100% of course.
YouTube High Definition for Firefox isn't just for video, it will also improve the music that you are listening to from YouTube as well by allowing the highest quality sound output adjusted automatically.
SuperYouTube for Chrome is a free extension capable of enhancing your viewing experience by allowing you to read comments while watching a video, scroll-over volume control, and more.